Analysis of Granular Flow in a Pebble-Bed Nuclear Reactor

TL;DR

This study uses large-scale simulations to analyze granular flow in pebble-bed nuclear reactors, providing insights into flow dynamics, mixing, and implications for reactor design.

Contribution

It presents the first detailed discrete-element simulation of pebble flow in realistic reactor geometries, including bidisperse cores and flow characteristics.

Findings

Flow is slow and well-characterized by velocity and diffusion profiles.

Little mixing occurs between different-sized pebbles in the core.

Wall friction significantly affects flow and residence times.

Abstract

Pebble-bed nuclear reactor technology, which is currently being revived around the world, raises fundamental questions about dense granular flow in silos. A typical reactor core is composed of graphite fuel pebbles, which drain very slowly in a continuous refueling process. Pebble flow is poorly understood and not easily accessible to experiments, and yet it has a major impact on reactor physics. To address this problem, we perform full-scale, discrete-element simulations in realistic geometries, with up to 440,000 frictional, viscoelastic 6cm-diameter spheres draining in a cylindrical vessel of diameter 3.5m and height 10m with bottom funnels angled at 30 degrees or 60 degrees. We also simulate a bidisperse core with a dynamic central column of smaller graphite moderator pebbles and show that little mixing occurs down to a 1:2 diameter ratio. We analyze the mean velocity, diffusion and mixing, local ordering and porosity (from Voronoi volumes), the residence-time distribution, and the effects of wall friction and discuss implications for reactor design and the basic physics of granular flow.